Humidity

1. Humidity • Absolute Humidity number of grams of water per cubic meter [ML-3] • Saturation Humidity maximum amount of water air can hold [ML-3] • Relative Humidity = % ratio of absolute humidity to saturation humidity.

2. Condensation • Condensation occurs when air mass can no longer hold all of its humidity. • Temperature drops => saturation humidity drops. • If absolute humidity remains constant => relative humidity rises. • Relative humidity reaches 100% => condensation => Dew point temperature.

4. Factors affecting Evaporation • Water temperature. • Air temperature above water layer. • Absolute humidity of air above water surface. • Wind – keeps absolute humidity low. - may increase the molecular diffusion. • Solar radiation – Langley = 1 cal./cm2; SI => joule/m2 = 4.18 x 104 Langleys.

6. Transpiration • Plants pump water from ground to atmosphere; accounts for most vapor losses in land-dominated drainage basin. • A function of plant density plant size limited by soil water. • Wilting point – surface tension of soil water interface > Osmotic pressure.

7. Evapotranspiration • Evapotranspiration = total water loss due to 1) free water evaporation, 2) plant transpiration, 3) soil moisture evaporation. • Potential evapotranspiration – the water loss, which occur if at no time there is a deficiency of water in the soil for the use of vegetation. • Actual evapotranspiration.

8. Limited soil-moisture storage Cool, moist Cool, moist Warm, dry

9. Ample soil-moisture storage

10. Formation of Precipitation • Humid air mass cooled to dew point temperature. • Condensation or freezing nuclei (clay minerals, salt, combustion products …). • Droplets coelesce to form raindrops. • Raindrops must be large enough such that they do not evaporate.

11. Precipitation – cont. • Adiabatic expansion P decreases => V increases =>T decreases • Dry lapse rate – Rising dry air 1OC/100m. • Wet lapse rate – Rising moist air 0.5OC/100m.

14. Effective uniform depth (EUD) of precipitation • Arithmetic mean method – the rain gauge network is of uniform density. • Isohyetal line method. • Thiessen method. - construct polygons - weighted by polygon areas

18. Events during Precipitation • Infiltration capacity (ability of soil to absorb moisture). - varies from soil to soil, from dry to moist. • fp = fc + (fo – fc) e-kt fp = infiltration capacity (L/T; ft/s or m/s). fc = equilibrium infiltration capacity. fo = initial infiltration capacity. k = constant (1/T; 1/s) (rate of decreased infiltration capacity)

19. Initial infiltration capacity Equilibrium infiltration capacity

20. All infiltrate some water always on the surface All infiltrate Puddles and overland flow

22. Water table • Water table = undulating surface at which pressure in fluid in pores = atmospheric pressure. Water table

25. Storm Hydrograph • Separate out: overland flow from baseflow. • D = A 0.2 • Or D = 0.827A 0.2 • D = number of days between storm peak and end of overland flow. • A = area of drainage basin (miles2; km2)

29. effluent influent

30. Effluent => Influent

33. Rainfall-Runoff Relationships • Rational equation: if it rains long enough, the peak discharge from a drainage basin is the average rate of rainfall times the area, reduced by a factor to account for infiltration. • Rational equation: peak discharge ~ (rainfall rate) x (drainage basin area).

34. Rainfall-Runoff Relationships • Q = CIA Q = peak runoff rate (L3/T; ft3/s, or m3/s) I = average rainfall intensity (L/T; ft/s; m/s). C = runoff coefficient (dimensionless). A = drainage area (m2 or ft2).

37. Hydrograph • Hydrograph – records discharge rate of a river at a single location.

38. Stream Gaging • Parabolic profile in velocity. • Q = VA • A = Cross sectional area. • qi = vidiwi • Q = q1 + q2 + q3 + … + qm • m = number of segments, usually 15-30.

39. Velocity profile in a river

41. Rating Curve • Rating Curve – discharge as a function of stream stage (elevation of the water surface above a datum).

43. Manning equation • V = 1.49 R2/3 S1/2 /n or R2/3 S1/2 /n • V – average velocity (L/T; ft/s or m/s). • R – hydraulic radius; or ratio of the cross-sectional area of flow in square feet to the wetted perimeter (L; ft or m). • S – energy gradient or slope of the water surface. • n – the Manning roughness coefficient.

44. Manning Equation

45. Duration Curves • Percent of time discharge was equal or exceeded. • P = 100 m /(n +1) • m = serial rank, where 1 = greatest flow. • n = number of data values.

46. low overland and return flows; high baseflow; strong water retaining (unconsolidated sand is thick). High overland and return flows; low baseflow; little water retaining (soils are thin).

47. Baseflow Recession

48. Q0

49. Baseflow recession • Q = Q0 e–at • Q = flow at time t after recession started (L3/T; ft3/s or m3/s). • Q0 = flow at the beginning of recession. • a = recession constant (1/T; d-1). • t = time since recession began. (T; d)

50. Recession begins